The present invention relates to a process for producing oriented hollow bodies of thermoplastic material, such as bottles and similar articles, of the type provided with a neck carrying an orifice collar, or finish, at its upper part for permitting the application of a sealing closure member, such as a cap or stopper.
It is already known to produce hollow bodies of thermoplastic material which possess substantially improved mechanical properties by resorting to production techniques leading to an orientation, preferably biaxial, of the macromolecules constituting their wall. According to a technique currently used for producing such hollow bodies, a first step involves the blowing of preforms of generally cylindrical shape, generally having a closed bottom and a neck molded into its final form, i.e. the form it will have in the finished article. Such preforms are then thermally conditioned in order to bring them to an optimal temperature to undergo orientation by stretching. Then, these conditioned preforms are blown in a final blowing operation in order to give them the form of the desired final article. The diameter and length of such a preform are selected to cause it to be substantially stretched, preferably biaxially, during the final blow molding.
It is indispensable that the hollow articles produced according to this technique be capable of being provided with a perfectly tight closure. It is for this reason that their necks are usually molded not during the final blow molding, but during the preform blow molding. In effect, since the thermoplastic material is at a relatively high temperature during blow molding of the preforms, the final neck can be molded at that time so as to assure a perfect reproduction of the desired configuration and a precise dimensioning of the orifice.
In contrast, it is not possible to mold the neck in a satisfactory manner during the final blow molding, which of necessity takes place at relatively low temperature, without resorting to extremely complex techniques such as those described in U.S. Pat. No. 3,651,186, issued on Mar. 21, 1972. It is for this reason that it is presently preferred to definitively mold the neck during preform blow molding.
Unfortunately, definitive molding of the neck during preform blow molding presents serious drawbacks. Thus, when the region of the neck is not axially symmetrical, for example because it is threaded, it is necessary to take special precautions to transfer the resulting preforms without damaging the neck and to correctly position them in the final blow mold. In addition, the thermoplastic material is inefficiently utilized at the level of the neck since the wall necessarily has a thickness almost equal to that of the starting tubular parison and has relatively poor mechanical properties since it is not oriented.